1. Field of the Invention
The present invention relates to electrical components for telecommunication systems.
2. Description of the Related Art
In a typical mobile telecommunication system, communication coverage is provided by base stations strategically located over the overall system coverage region. Each base station supports communications with the mobile units currently located within its coverage area. Forward-link signals intended for the various mobile units located within the base station coverage area are combined for transmission from the base station as a single combined forward-link signal. The individual mobile units receive and process the combined signal to extract only the appropriate corresponding signal. For example, in a code division multiple access (CDMA) telecommunication system, the signals for different mobile units are distinguished by unique Walsh codes that are used to encode the various signals. Each Walsh code corresponds to a different channel. Channels are dynamically assigned to individual mobile units within the coverage area as they are needed to support communications between the base station and the mobile units. In conventional CDMA systems, all of the processing (e.g., encoding, modulation, demodulation, decoding) for each different channel is handled by a separate component called a cell-site modem (CSM) located at the base station. Each CSM handles a different CDMA channel.
In a typical CDMA system, each cell site (or cell-site sector) can support up to 64 different channels. As such, a base station can be configured with up to 64 different CSMs, with each CSM assigned to handle a different one of the 64 CDMA channels. For forward-link processing, where all of the forward-link signals for the various CDMA channels are combined for transmission as a single combined forward-link signal, the forward-link signals generated by the different CSMs are combined to form the single combined signal for transmission. One possible base station architecture would have a single adder connected to receive the outputs directly from all of the different CSMs to generate the combined signal for transmission. When there are as many as 64 different CSMs, such an implementation would require a very large adder that makes this architecture undesirable for various reasons (e.g., cost, size, and power consumption).
FIG. 1 shows a block diagram of an alternative forward-link architecture for the various CSMs in a base station of a typical CDMA system. According to the architecture of FIG. 1, N boards are linked together in a daisy-chain configuration (i.e., with the output of each board being added to the output of the immediate downstream board). Within each board are two columns of CSMs 102 with the CSMs of each column linked together in a daisy-chain configuration (i.e., with the output of each CSM being added to the output of the immediate downstream CSM).
At the CSM level, each intermediary CSM in a column (i.e., not the first or last CSM in a column) receives the signal output from the CSM immediately upstream, adds its own signal, and transmits the augmented signal to the CSM immediately downstream. The first CSM in a column simply transmits its own signal to the second CSM. The last CSM in a column receives the signal output from the CSM immediately upstream, adds its own signal, and transmits the resulting signal corresponding to the contributions from all of the CSMs in that column to a special adder 104, which receives the cumulative signals from both columns and generates a cumulative board signal.
The adder in the first board in the board-level daisy chain (i.e., Board 1 in FIG. 1) receives only the two cumulative column signals from that board to generate the cumulative board signal for Board 1. In addition to the two cumulative column signals, the adder in every other board (i.e., Boards 2 to N) also receives the cumulative board signal from the board immediately upstream. As such, the cumulative board signal generated by each board corresponds to the sum of the signals from all of its CSMs plus all of the signals from all of the CSMs in upstream boards. In this way, the signal generated by the adder in the last board in the daisy chain (i.e., Board N) corresponds to the sum of the signals from all of the CSMs in the base station.
Of course, other architectures are also possible. For example, instead of two columns of daisy-chained CSMs per board, a board may have only a single daisy-chain of CSMs or alternatively more than two columns of daisy-chained CSMs. Similarly, the boards need not be daisy-chained together. In an alternative embodiment, all of the board outputs can be combined at a single adder.
The use of daisy-chain connections both at the CSM level (within columns) and/or at the board level (between boards) avoids the need for large, expensive, power-hungry adders to added many different signals together at once. One drawback in the architecture shown in FIG. 1, however, is that it is susceptible to catastrophic single-point failures. In general, at the CSM level, a single-point failure of any given CSM would result in the loss not only of the signal from that CSM, but also of the signals from all upstream CSMs within the same column. In the worst case, if the last CSM in a column were to fail (i.e., break), none of the signals from any of the CSMs in that column would make it to the adder and would therefore be omitted from the final combined signal. Similarly, at the board level, a single-point failure of any given board would result in the loss not only of the signals from that board, but also of the signals from all upstream boards. In the worst case, if Board N in FIG. 1 were to fail, none of the signals from any of the CSMs in the base station would make it to the antenna for transmission.